Pressurized water nuclear reactors generally include a core consisting of fuel assemblies of prismatic form which are disposed vertically and juxtaposed. The core is placed within the vessel of the nuclear reactor, in which vessel water for cooling the reactor circulates. The core is surrounded by a partitioning which closely follows the form of its peripheral surface and ensures retention thereof within a core envelope of cylindrical form with a vertical axis. The partitioning comprises vertical plates placed at right angles to one another and reproducing the external form of the core, as well as reinforcements consisting of horizontal plates disposed at regular intervals according to the height of the core, within an annular space provided between the periphery of the core and the internal surface of the core envelope.
In the pressurized-water nuclear reactors of the most current type which were constructed up to the beginning of the 1970's, cooling water penetrates into the core envelope through one or more orifices provided in its upper part and circulates from top to bottom within the annular space surrounding the core, the horizontal reinforcement plates of the partitioning being pierced, with the exclusion of the plate situated uppermost and disposed above the orifice of the core envelope, which constitutes the upper reinforcement. The cooling water circulates within the core in the vertical direction and from bottom to top. On either side of the partitioning, the cooling water therefore circulates in opposite directions. On the other hand, this cooling water in circulation suffers losses of head, in particular within the assemblies of the core, at the location of the bracing plates ensuring the retention of the fuel rods. A pressure difference therefore exists on either side of the vertical plates of the partitioning, which pressure difference is neutralized in the lower part of the core where the current of cooling water penetrates into the core and which is at a maximum at the upper part of the core, the cooling water in circulation having then suffered the maximum loss of head.
The vertical plates of the partitioning are, on the other hand, mounted at right angles to one another, without the junction zone being closed off in a sealed manner. There is therefore, in general, an interstice between two successive partitioning plates, this interstice placing the annular space at the periphery of the partitioning in communication with the core disposed within the partitioning.
On account of the pressure difference existing between the two sides of the vertical walls of the partitioning, parasitic jets of cooling water appear when the nuclear reactor is in service. These parasitic jets of water are directed in the direction extending from the annular space around the partitioning towards the interior of the core and are liable to strike the fuel rods of the assemblies disposed in proximity to the zones of connection of the partitions. These jets give rise to vibrations of the rods, which are detrimental to their proper maintenance in service.
In nuclear reactors of more recent design, an attempt has been made to reduce or to eliminate these parasitic jets, by reducing or by neutralizing the differential pressure on either side of the partition or even by making this pressure difference negative.
In order to achieve this result, the structure and design of the core envelope and of the partitioning were slightly modified in order to obtain a circulation of the cooling water of the reactor directed from bottom to top, both in the core and in the annular space located at the periphery of the partitioning. The core envelope does not include any orifice at its upper part, and the horizontal upper reinforcement of the partitioning is pierced in order to permit the passage of the cooling water, at the upper part of the annular space. The horizontal reinforcements of the partitioning are disposed in coincidence with the bracing plates of the assemblies of the core, and these reinforcements are pierced in such a manner that the cooling water circulating within the annular space suffers a loss on passing through the reinforcements which is equivalent to the head loss suffered by the water in circulation within the core, on passing through the bracing plates of the assemblies.
In nuclear reactors of older design which have been placed in service and utilized according to the principle of the circulation of the cooling water in countercurrent on either side of the partitioning, an intervention has been provided permitting the cooling water to be caused to circulate in the same direction on either side of the partitioning of the core. This intervention consists in blocking the orifice or orifices located at the upper part of the core envelope and in piercing the upper reinforcement of the partitioning.
Such an intervention produces a certain improvement of the operation of the reactor, by a reduction of the intensity of the parasitic jets of cooling water through the partitioning. However, there are still zones, in particular in the bottom part of the core, where the pressure within the annular space at the periphery of the partitioning is greater than the pressure within the core. This is due, in particular, to the fact that the reinforcements are not generally disposed at the same level as the grids of the assemblies of the core.
In the case where the play between the partitions is relatively great, the parasitic jets of cooling water exhibit locally an intensity which exceeds generally accepted limits. In this case, it is necessary to reduce the play between the partitions, either by hammering of the joint or by screwing of the parts of the partitions which are in coincidence.
Such operations must be carried out under water during a shutdown period of the reactor and require the utilization of specially designed means.